Effect of Precursors on Trimetallic Ruthenium-Based Catalysts Supported on γ-Al2O3 Pellets for Low-Temperature Ammonia Decomposition

Abstract

Ammonia is a promising candidate as a liquid hydrogen energy storage medium, but it requires catalytic decomposition (ammonia cracking) to regenerate hydrogen. Recently developed trimetallic ruthenium–potassium-promoter (RuKM) ammonia decomposition catalysts have exceptionally low ammonia decomposition temperatures, able to perform the decomposition as low as 250 °C, which is significantly lower than other known catalysts that require temperatures above 500 °C. However, the effects of the RuKM precursor on the catalytic activity have not been investigated. We report the observed differences of 3% ruthenium/12% potassium/1% yttrium (RuKY) catalysts on γ-alumina synthesized from chloride-, nitrate-, and acetate-based precursors. Catalysts synthesized from chloride-based precursors demonstrated the lowest ammonia decomposition catalytic activity at lower reaction temperatures. In contrast, those synthesized from nitrate-based precursors demonstrated the highest yield, despite similar metal loading. This difference in reactivity is most apparent between 250 and 400 °C, as the conversion rates of the catalysts synthesized with chloride-free precursors are up to 50% greater than those synthesized with chloride precursors. The observed differences in catalytic activity were much less apparent above 450 °C. The observed activation energies of the catalysts were independent of the precursor utilized, despite the difference in catalytic activity, suggesting that the active site composition was the same for all catalysts. These results suggest a pathway to improved ammonia cracking catalysts by tailoring the precursor used in the synthesis.